Abstract [en]

Density dependence (DD) is a central concept in population ecology and in the management of harvested populations. For example, DD underpins the idea of additive versus compensatory mortality and is a tenet in the paradigm of resource limitation and regulation. Yet the prevalence and importance of DD remains disputed in most organisms, including ducks, which are focal in game management, conservation and zoonotic diseases. Based on 154 data entries from 54 studies in the peer-reviewed literature, we here synthesize and evaluate the prevalence of DD in breeding ducks in relation to (1) species and guild (dabbling versus diving ducks), (2) stage in the breeding cycle (nesting, duckling, recruitment) or, alternatively, in terms of population dynamics, (3) study type (descriptive/nonmanipulative versus experimental), (4) continent (Europe versus North America), (5) spatial level (wetland, landscape, regional, continental) and (6) biome (tundra, boreal, nemoral, prairie, mediterranean). One conclusion from this review is that it is difficult to find general patterns about the prevalence of DD unless data are broken down to subsets, for example, to stage or spatial level. With respect to stage, DD effects occur at all stages of the breeding cycle. During the nesting and duckling stages, the frequency of cases detecting versus not detecting DD is roughly the same. However, in cases referring to the recruitment stage, i.e. to survival of fledged ducks until 1 year old at the most, DD was the rule, suggesting that DD processes may operate mainly outside the breeding season. Further subdivision of data shows that spatial scale is important to the prevalence of DD in nesting ducks—rare on the wetland level and more common on higher spatial levels. In studies of population dynamics (i.e. based on time series data only), DD was more often found in diving than in dabbling ducks. This corroborates previous suggestions that dabbling ducks largely should be considered as r-selected species, in contrast to more K-selected diving ducks, which start to reproduce at an older age and often breed in more stable wetland environments where resources may be easier to track and populations thus often are closer to carrying capacity. However, the picture of DD in ducks is far from complete, and knowledge gaps for future studies to address include: (a) data from Russia, which holds a large part of the breeding ducks in the Northern hemisphere, (b) experimental studies on more species to separate density-dependent factors from other drivers of population change and to tease apart spatial and temporal interactions in the underlying processes, (c) time series analyses addressing population dynamics, especially from outside North America, (d) studies relating duck numbers to limiting resources, which arguably is the most relevant measure of density, (e) the timing of DD processes in relation to harvest and natural mortality.